Thermoplastic polymers have heretofore been foamed by imbibing or impregnating the polymer with a suitable blowing agent and subsequently heating the imbibed polymer by conventional means such as steam or forced air ovens and the like. Thermoplastic polymers which have been blended with a polarizing agent and which have been impregnated or imbibed with a suitable blowing agent have been foamed by subjecting the resultant treated polymers to microwave energy.
It would be desirable to be able to foam thermoplastic polymers which have been impregnated or imbibed with a suitable blowing agent to electromagnetic radiation without also having to blend into the polymer a polarizing agent.